1. Field of the Invention
This invention relates to a process for producing propylene, hexenes and, optionally, ethylene, by metathesis from butenes and pentenes and then aromatizing the hexenes to benzene.
2. Description of the Prior Art
Olefin metathesis is the catalytic double disproportionation by cleavage and reformation of the carbon double bond (C═C) of two olefin compounds to form two different olefin compounds. This reaction is reversible, and the overall product distribution is limited by a thermodynamic equilibrium. The catalysts are generally homogeneous or heterogeneous transition metal compounds, particularly compounds of transition metal Groups VI to VIII.
U.S. Pat. No. 6,646,172 discloses a multistage process for preparing propylene from metathesis of butenes and from separate metathesis of the C5 alkenes produced in the metathesis of butenes.
U.S. Pat. No. 6,580,009 discloses a process for preparing propylene and hexene by metathesis of a raffinate II feedstream of olefinic C4 hydrocarbons and ethene with fractional distillation to recover butanes, butenes and a pentene-containing fraction for recycle. This patent also discloses methods for the removal of butadiene, acetylenic compounds, and oxygen-containing impurities from a C4 feedstream.
U.S. Pat. No. 6,538,168 discloses a process for preparing C5-C6 olefins by metathesizing butenes into a mixture of C2-C6 olefins and butanes, separating C2-C3 olefins from C4-C6 olefins, further separating C4-C6 olefins into hexene and methylpentene and a recycle of butanes, butenes, pentene and methylbutene.
U.S. Pat. No. 6,777,582 discloses a process for metathesis of 1-butene and 2-butene to propylene and 2-pentene which is recycled with product butenes to foam propylene and hexene.
Aromatization of alkenes is a process of dehydrogenation, cyclization and aromatization of the alkene. The catalyst for this process must be multi-functional to have an acceptable conversion and selectivity for the desired products. Aluminosilicate zeolites are known catalysts for aromatization. Some zeolite catalysts for aromatization contain a Group VIII deposited metal, such as platinum, and elements other than silicon and aluminum, such as germanium, in the zeolite crystalline framework.
U.S. Patent Application Publication no. 2008/0255398 discloses a process for the aromatization of C6 to C12 alkanes, such as hexane, heptane and octane, to aromatics, such as benzene, ethyl benzene, toluene and xylenes, with a non-acidic aluminum-silicon-germanium MFI zeolite on which platinum has been deposited. One catalyst for the aromatization of C6 to C12 alkanes is Pt/CsGeZSM-5.
Metathesis of butenes is known in the prior art. It is known that metathesis of 1-butene, 2-butene and isobutene will yield propylene, a desirable product which can be further processed into other compounds, such as acrylic acid, acrylonitrile, isopropanol, propylene oxide, and into other products, such as polypropylene. Commercialization of metathesis of butenes has been impeded due to the lack of uses for the other products of the metathesis reaction, such as hexene and higher olefins. Since metathesis is an equilibrium reaction, these products which have been less desirable than propylene are produced in an amount beyond what is useful for their available market. It would be advantageous to convert these products to more marketable products.